1. Field of the Invention:
This invention relates to corrugated cardboard manufacturing methods and specifically to devices for producing double-free corrugated paperboard or cardboard webs formed by laminating flat facing webs to opposite sides of a corrugated or fluted paper web.
2. Description of the Prior Art:
Corrugated paperboard is manufactured at high production rates on corrugator machines which are well known in the paper industry. A typical machine includes a corrugating and glueing section, a heating section, and a cooling section. In the first section, corrugations are formed transversely across an intermediate web and liquid adhesive is applied to the tips of the flutes of the corrugated web or medium. After the adhesive is applied, a first single-face liner web is brought into contact with the glue-coated flutes to form a laminated single-face web consisting of one liner and a corrugated medium. The device which performs this operation is typically referred to as a single-facer. The single-face web is then advanced past a glue machine downstream to apply adhesive to the exposed flute tips of the medium and thereafter a second double-face liner web is applied to the exposed side of the corrugated medium. The combined and double-face web consisting of a single-face web and the second liner then passes through a heating section where the liquid adhesive holding the second liner to the corrugated medium is cured. The adhesive is cured by passing the freshly glued web across a series of hot plates under pressure from above. The hot plates are usually heated internally by steam to a temperature necessary to cure the adhesive. The pressure is provided by moving the web over the hot plates under an endless ballast belt which rests upon the upper liner of the single-face web and advanced together with the web at the same speed. Weight rollers on top of the lower flight of the belt provide additional pressure to hold the web laminates together and maintain them flat against the hot plates to enhance heat transfer from the hot plates to the web for curing purposes. As the heat acts upon the adhesive, it also drives moisture out of the combined web so that the finished corrugated paperboard web exits from the downstream end of the heating section in a stiffened substantially flat condition. The web then passes immediately through a cooling section to reduce its temperature prior to being divided into a plurality of webs of selected widths each of which is then cut transversely to form corrugated blanks.
It happens relatively often that the moisture of the individual layers of the corrugated cardboard is not uniform over the width thereof. One explanation is that moisture seeps into the paper rolls and penetrates the outer layers of the paper on the rolls but only to a certain depth. Once installed into a corrugator the roll of paper having differing moisture contents over the width of the paper web contributes to warping of the final product. Other factors which may contribute to warping are stresses placed on the web by machinery, adhesive quality and quantity, and heat transfer characteristics.
It is helpful to understand that a warped blank is not flat, and includes a slight curl across the surface of the blank.
The adhesive commonly used in the process is an ungelatinized granular starch in a liquid carrier that is cured by gelatinization and dehydration which results from the application of heat. Prior art devices have suggested the application of moisture and heat to the paper web to precondition the web prior to the application of the adhesive to the fluted medium.
A variety of approaches to improving corrugated cardboard for manufacturing processes have been developed. The patent to McDonald et al., U.S. Pat. No. 3,892,613, notes the importance of moisture and temperature conditioning of the corrugated medium and moisture control of the liner.
Hayasi et al., U.S. Pat. No. 3,829,338, discloses a double-facer machine heat control in which the temperature of the bottom liner is measured at one or more locations along the double-facer heater section, and in which a comparator control device compares the measured value with a predetermined set point. Based upon the comparison, the effective weight or number of ballast rollers engaging the lower run of the double-facer belt is appropriately regulated to attain optimum liner temperature.
The device disclosed in Lawton, U.S. Pat. No. 4,042,446, is an apparatus for supplying heat and pressure to a corrugated paperboard and includes a temperature sensing device to measure the temperature of the corrugated web leaving the heating section of the apparatus. The weight bearing down upon the web is adjusted automatically via rollers contacting the belt in response to the measured temperature in order to maintain the temperature at or near a predetermined value.
Schommler, U.S. Pat. No. 4,556,444, describes a heating device for applying heat to corrugated cardboard which includes several heating plates spaced in the feeding direction of the web and covering the maximum width of the corrugated cardboard web. A moisture sensor is placed in front of each heating plate in the direction of movement. A control device monitors the moisture content of the web and heat is applied to the plates to drive moisture out of the web in order to equalize the moisture content across the web.
Kruglinski et al., U.S. Pat. No. 3,654,032, discloses an apparatus which adds moisture to the outer sheet after the corrugated board leaves the heating plates to equalize moisture content of the sheets and prevent warpage. Kruglinski et al. also discloses sensors for measuring moisture content of the web at the input end of the machine. Information gleaned from the moisture sensors determines the amount of moisture which is introduced into the web.
Thayer et al., U.S. Pat. No. 3,981,758, discloses a method and apparatus for improving overall quality and reducing warp in corrugated paperboard blanks. Generally stated, the device and method of Thayer automatically control the time of heat exposure, bonding pressure, web or liner tension while entering the heater section, and application of moisture to the web.
A primary requirement for proper operation of a corrugator device is that the moisture content of the paper rolls which supply the web to the machine is less than eight percent to make good quality corrugated cardboard. As is shown by the prior art, two methods are most often used to control the moisture content. The first is to apply heat by way of hot plates, and the second method is to apply moisture to equalize the moisture content across the web. A device well known in the industry which incorporates the second technique is a Gaylord Shower device. However, a system which adds moisture to the web is ineffective when the moisture content of the web is already too high to make quality corrugated cardboard. The hot plate technique of removing moisture from various areas of the web is ineffective in that if the heat is supplied to the hot plates via electrical heating or steam supply the amount of time required to vary the process parameters can result in hundreds of linear feet of wasted corrugated board before the process is stabilized and the variances in web moisture are eliminated.
An improved device and method for controlling the moisture content across the web of a corrugated cardboard manufacturing apparatus which also includes process monitoring sensors to improve efficiency and reduce waste is needed.